1. Field of the Invention
The present invention relates to liquid crystal-based video and graphics display devices, and, in particular, to voltage amplifiers for such devices.
2. Description of the Related Art
A substantial need exists for various types of video and graphics display devices with improved performance and lower cost. For example, a need exists for miniature video and graphics display devices that are small enough to be integrated into a helmet or a pair of glasses so that they can be worn by the user. Such wearable display devices would replace or supplement the conventional displays of computers and other devices. In particular, wearable display devices could be used instead of the conventional displays of laptop and other portable computers. Potentially, wearable display devices can provide greater brightness, better resolution, larger apparent size, greater privacy, substantially less power consumption and longer battery life than conventional active matrix or double-scan liquid crystal-based displays. Other potential applications of wearable display devices are in personal video monitors, in video games and in virtual reality systems.
Miniaturized displays based on cathode-ray tubes or conventional liquid crystal displays have not been successful in meeting the demands of wearable displays for low weight and small size. Of greater promise is a micro display of the type described in U.S. Pat. No. 5,596,451 of Handschy et al. (digital pixel driver) and in European patent application no. 98122934.7 (publication no. EP 0 953 959 A2), of Walker et al. (analog pixel driver), the disclosures of which are incorporated into this disclosure by reference. This type of micro display includes a reflective spatial light modulator that uses a liquid crystal (LC) material as its light control element. Typically, a ferroelectric liquid crystal (FLC) material is used as the light control element.
To drive the pixels of the spatial light modulator, there is a need for an inexpensive low-power circuit that amplifies a single-ended voltage input and shifts the offset of the output to a stable voltage. Preferably, such a circuit would permit one to adjust the gain and offset values by changing the values of only two resistors. Furthermore, the overall amplifier would have a 3 dB bandwidth that is independent of the gain. Additionally, the power dissipation of the AC signal path would be independent of the gain.
Allen describes an elegant solution that has these properties. See P. Allen and M. Terry, xe2x80x9cThe Use of Current Amplifiers for High Performance Voltage Applicationsxe2x80x9d, IEEE Journal of Solid State Circuits, vol. SC-15, no. 2, pp. 155-162, April 1980. Allen uses a high-gain bipolar current amplifier with resistive feedback. However, the high-gain current amplifier Allen employs relies on base-current multiplication to achieve high gain. This is realizable in bipolar technology, but not in CMOS.
Van de Plassche describes an instrumentation amplifier that employs a voltage-to-current converter at the input stage and that is realizable in CMOS. See R. Van de Plassche, xe2x80x9cA Wide-Band Monolithic Instrumentation Amplifierxe2x80x9d, IEEE Journal of Solid State Circuits, vol. SC-10, no. 6, pp 424-431, December 1975. The Van de Plassche implementation is not suitable for use to drive the pixels of the spatial light modulator, however, because it is optimized for a differential input and is designed to reject common-mode voltages. It is preferable to use a single-ended input to drive the pixels of the spatial light modulator. Moreover, when driving the pixels, it is preferable to control, not reject, the DC values and currents in the amplifier circuit.
Thus, it can be seen that modern liquid crystal pixel driving techniques impose contrast fidelity and production cost limits upon LC-based micro displays, and hinder the use of these micro displays in many applications.
Therefore, there is an unresolved need for an improved liquid crystal pixel driving technique that can increase LC-based micro display contrast fidelity and lower production costs.
A liquid crystal pixel driving technique is described that uses a voltage amplifier employing operational transconductance amplifiers and current mirrors to increase LC (Liquid Crystal)-based micro display contrast fidelity and lower production costs.
The voltage amplifier has a current follower coupled between a voltage-to-current converter and a voltage buffer. The voltage amplifier employs operational transconductance amplifiers and at least one current mirror to achieve a fixed output swing of an output voltage from the voltage buffer based upon an input swing of a single-ended input voltage to the voltage-to-current converter. A marginal change in the output voltage from the voltage buffer is substantially linear and proportional to a marginal change in the input voltage input to the voltage-to-current converter.
For one embodiment of the voltage amplifier, gain and voltage offset of the output voltage can be adjusted by changing the values of two resistors. At least one of these two resistors may be an off-chip resistor.
For one embodiment of the voltage amplifier, the voltage-to-current converter and the current follower together use two operational transconductance amplifiers and two current mirrors. For an alternative embodiment of the voltage amplifier, the voltage-to-current converter and the current follower together use three operational transconductance amplifiers and a current mirror.
For one embodiment, the voltage amplifier is used in an analog driver for at least one of the pixels of an array of pixels in a spatial light modulator. The spatial light modulator, in turn, can be used in a display device.
Overall, the voltage amplifier can provide a 3 dB bandwidth which is independent of the gain. Moreover, power dissipation of the AC signal path is independent of the gain.